One of the data gathering methods presently used for MRI is known as the "Spin-Echo" method. In this method, as in the other MRI methods the object or subject is placed into a strong static magnetic field to align its normally random "Spins" (nuclei exhibiting magnetic moments) with the magnetic field to form a "spin or magnetic vector". Radio frequency pulses at Larmor frequencies are used to change the state (i.e. the magnetic alignment) of these "spins" and the free induction decay ("FID") signals brought about by the return of the spins to the aligned state corresponding to the strong static magnetic field are used for generating the image. The Larmor frequency signals are transmitted during the imposition of gradient pulses to enable determining the location of the sources of the FID signals in the subject being imaged. Thus, facilities are available for applying magnetic gradients in "X", "Y" and "Z" orthogonal directions.
In the "Spin Echo" method, a pulsed magnetic field rotating at the Larmor frequency is first applied to nutate the spin vectors 90 degree from the aligned position (hereinafter "a 90 degree RF pulse"). When the spins or vectors are 90 degrees away from the aligned position, they characteristically dephase. In effect, in the frame of the rotating field the magnetic vector dephases into a plurality of vectors rotating about the axis of the the strong magnetic field in clockwise and counter-clockwise directions simultaneously. Certain of the vectors rotate faster then others. A second pulsed magnetic field is applied to now nutate the spins 180 degrees, (hereinafter "a 180 degree RF pulse"). An echo is obtained a certain time period after the spins are nutated 180 degrees. The 180 degree nutation provides a mirror-like reflection of the dephasing spin vectors. In the frame of the rotating field, the nutated spin-vectors continue to rotate in their respective clockwise and counter clockwise directions; thus, the faster dephasing vectors catch up with the slower dephasing vectors and there is a refocusing of the spins at a time Ta equal to the time Ta between the aforementioned 90 degree RF pulse and the 180 degree RF pulse.
Presently the spin-echo method is accomplished by applying the 90 degree RF pulse during the application of a gradient pulse, such as a "Z" gradient pulse, for example. Subsequently, the 180 degree RF pulse is applied at a time Ta after the 90 degree RF pulse and also during the application of a similar pulse of the same gradient. The application of the gradients results in a selection of a specific slice in the subject to be imaged.
Encoding gradient pulses such as for example "Y" gradient pulses, are applied for example, between the transmission of the 90 degree and the 180 degree RF pulses. A third gradient such as an "X" gradient read pulse is applied after the application of the 180 degree RF pulse. It is during the read pulse that the echo is received. Several echos can be obtained by applying subsequent 180 degree pulses with slice selecting gradient pulses and gradient read pulses during the echo acquisitions after time periods Ta until the echo is too small to provide a meaningful reading. This procedure gives an image in a slice or a plane.
The plane is selected by the choice of the slice selecting gradient. Thus, when a "Z" slice selecting gradient is used, which conventionaly is a gradient in the direction of the large static magnetic field, a plane normal to the large static magnetic field is selected. As used herein, the "X" and "Y" coordinates are orthogonal to the direction of the large static magnetic field with "Y" in the direction of the encoding pulses and "X" in the direction of the reading pulse.
Appropriate permutation of the roles of the gradients enables the imaging of slices facing other directions as well. The encoding and the read gradient can be inter-changed within the scope of the invention.
In imaging, in general, the scientists are always endeavoring to increase the spatial resolution and lower the time required to provide the image. These are contrary aims; that is decreasing the time generally decreases the resolution. Thus, a method for decreasing the time while maintaining the same resolution or a method for increasing the resolution while imaging during the same time period is highly desirable. In MR imaging, increasing the time of acquiring an image does not pose any danger to the patient because there is no dangerous radiation being used; nonetheless, since throughput is an important consideration economically, clinicians and imaging scientists are always interested in decreasing the time required for acquiring images. In some cases the time saved might be used for accumulating of several images of the same slice and subsequent averaging resulting in an increase in the signal-to-noise ratio.
A further goal desired by imaging scientists is to be able to zoom during the acquisition stage. In other words, during the imaging process if a particular portion of the body shows an interesting manifestation; it is often desirable to zoom in on this manifestation and to thereby focus on the manifestation to the exclusion of other data. This is presently accomplished in MRI systems as a computer step after the acquisition of the data, if the imaging is to be accomplished within the same time frame. However no increase of the spatial resolution can be achieved by such manipulation of the data. It would be desirable to be able to zoom during the acquisition of data. Such zooming would increase the resolution of the portion of the image focused upon in a natural manner. A prior art problem encountered when zooming during the acquisition of data is that "aliasing" artifacts are generated unless the number of encoding cycles is increased with a proportional increase of the total acquisition time. Aliasing is basically caused by undersampling. Thus, focusing on a smaller section of the body being imaged while using the samplings of the larger image results in aliasing artifacts.
Accordingly, an object of this invention is to provide improved slice selection for MRI systems using spin-warp echoing data acquisition and two dimensional Fourier transform data processing imaging techniques. A synergistic advantage of the slice selection method is that controlled zooming is provided during the data acquisition process substantially without aliasing artifacts.
Accordingly, an improved slice selection method for magnetic resonant imaging systems is provided, said method including the step of:
obtaining images substatially free of aliasing artifacts even while reducing the extent of the image slice in the encoding direction below the size of the object.
A feature of the present invention is the application of the system to provide zooming during data acquisition rather then relying on after-acquisition data manipulation.
A further feature of the invention provides for applying the 90 degree RF pulse in the spin-warp echo technique while a gradient pulse is applied in the encoding direction as opposed to a gradient pulse being applied in the slice selection direction. A 180 degree RF pulse is subsequently applied during the application of a slice selecting gradient pulse. Alternatively, the slice selecting gradient is applied during the 90 degree RF pulse, and the 180 degree RF pulse is applied while a pulse in the encoding direction is applied. A read gradient pulse orthogonal to both slice selection direction and the encoding direction is applied during the echo pulse.